1. Field of the Invention
The present invention relates generally to voltage sensing devices, and in one of its aspects, to an input circuit for an annunciator.
2. Description of the Prior Art
An industrial annunciator monitors abnormalities within some system such as a refinery, chemical plant or power plant. Sensing devices such as mercoid switches detect when some abnormality has occurred. The abnormality could be too high a temperature in a boiler, or loss of pressure in a fluid line, to a dangerous flow rate. Often in the past, voltage sensing devices have been made up of transformers and resistive networks which allow considerable current to run through the sensing device itself to the system being monitored. Normally, many of the points to be monitored would be series contacts in the line supplying power to a motor or other system to be monitored. In order to limit the total amount of current going to the motor for example so that the motor would not still be supplied enough current to operate when a single contact was open, the resistance of the sensing device was made fairly high, but by making the resistance of the sensing circuit higher, there often was not sufficient current to actuate the sensing circuits when several of the series contacts were open. The total number of sensing devices that could be operational at any one time was thus fairly limited, typically three or four.
U.S. Pat. No. 4,021,683 issued to Splatt and U.S. Pat. No. 4,065,669 issued to Bogel both show the use of diode rectifier bridges and optical couplers in switching circuits. Both of these circuits are literally and functionally in series with the load in normal operation. Splatt, for instance, is designed to control the supply of alternating current to the load.
Various devices have beeen proposed for monitoring or sensing some event that has happened elsewhere in a circuit. It is common in such monitoring or sensing devices which use optical couplers for electrical isolation purposes to try to protect the light emitting device which is usually a light emitting diode (LED) from excessive voltage or current by using a resistor in series with the LED. The series resistor-LED combination is then combined with a zener diode facing in the opposite direction from the LED as shown in U.S. Pat. No. 3,848,140 issued to Guermeur et al. and U.S. Pat. No. 3,967,257 issued to Hager in order to protect the LED from excessive reverse bias voltage in one direction and excessive current in the other. Another arrangement to protect the LED is to include a plurality of series diodes in parallel with the resistor LED series combination, facing the same direction as the LED as shown in U.S. Pat. No. 4,121,122 issued to Pokrandt and Canadian Patent No. 958080 issued to Oulton. This arrangement increases the sensitivity of the LED for testing low levels of current, but is also designed to protect the LED from excessive current. In all of these cases, however, it is the current to the LED that is being limited and the intent is to protect the LED.